Recently, ionomer resin have widely used for cover materials of golf balls (e.g. Japanese Patent Publication No. 49-27093 etc.). This is because the ionomer resin is superior in durability, cut resistance, rebound performance, etc. and it is easily processed.
However, since the ionomer resin has high hardness and rigidity properties, the resulting golf ball is poor in shot feel and controllability (ease of exhibiting spin) in comparison with balata (transpolyisoprene) which has been used as a cover material of a multi-layer structure golf ball containing a thread rubber layer.
In order to improve shot feel and controllability of ionomer-covered golf balls, softening the ionomer resin has been intensively studied.
For example, Japanese Patent Laid-Open Publication Nos. 1-308577 and 5-3931 suggest to blend or mix a hard ionomer resin with a soft ionomer resin prepared by neutralizing a terpolymer of .alpha.-olefin, an unsaturated carboxylic acid (e.g. acrylic acid, methacrylic acid, etc.) and an acrylate with sodium or zinc ions, thereby improving shot feel and controllability.
However, the blending of the soft ionomer resin as suggested by the above mentioned publications has not provided with shot feel properties which are closed to the balata covered golf balls. Instead, the rebound performance and flight performance are adversely lowered by blending the soft ionomer resin and, therefore, sufficient technical effects have not been obtained.
Japanese Patent Laid-Open Publication No. 5-220240 suggests to improve shot feel and controllability by blending the ionomer resin with a polymer containing glycidyl groups. According to this method, shot feel and controllability are slightly improved but they are not sufficient and ionomer-covered gold balls having sufficient characteristics (particularly, rebound performance, flight performance, etc.) are still desired.